Electronic mail has become a commonly used and widely accepted means of communication. As a result, it is not unusual for some individuals to send and receive more than a hundred electronic mail messages in a single day. This has led to an explosion in the size of the memory required for many electronic mail accounts. For example, there are some corporate users whose mailboxes routinely exceed 50 Mbytes in size and for whom a single message with one or more attachments will often contain 5 Mbytes or more of data. Consumers also use electronic mail (or other techniques for routing electronic data files) to exchange music and other multi-media files, and these files will frequently contain multiple Megabytes of data.
As the use of electronic mail continues to grow, so does the need to access electronic mail accounts from a variety of locations. For example, many now use portable electronic communication devices for sending and retrieving electronic mail messages. These portable communication devices, which include wireless telephones, personal digital assistants, specialized portable electronic mail messaging devices, and even laptop computers, allow users to conveniently access their electronic mail accounts from almost any location, without being tied down to a stationary personal computer or network workstation.
In order to use these portable communication devices to retrieve electronic mail from one or more existing electronic mail accounts, however, the user must typically set up a new electronic mail account, specific to the device, with a service provider. The service provider must then retrieve the electronic mail files that make up the contents of the user's existing electronic mail account (i.e., the user's “regular” electronic mail account) or accounts, and store these contents in the user's electronic mail account or accounts for the portable electronic device (i.e., the user's “portable” electronic mail account or accounts). The user can then access this synchronized portable electronic mail account from the portable communication device.
Similarly, many now use aggregate electronic mail accounts to simultaneously access a variety of other existing electronic mail accounts. Again, the service provider for the aggregate electronic mail account must retrieve the contents of the user's existing regular electronic mail accounts, and synchronize these contents in the user's aggregate electronic mail account.
With a multi-tiered Web services architecture (e.g., an architecture made up of storage, networking, front-end servers and application servers), however, this synchronization process typically requires the underlying system to scale at the storage tier, which is usually the most expensive in terms of resources. Thus, in order to make this synchronization service cost effective, the service provider will generally: (a) place a strict limit on the amount of storage memory available to the user to maintain his or her portable electronic mail account (e.g., between 2–5 Megabytes is common), or (b) charge the user a premium for the right to use more than a threshold amount of storage memory (e.g., $10–$20/month for each extra 100 Megabytes of storage memory).
When the amount of available storage is strictly capped, a user may be denied access to a particular mailbox or to a particular message within a mailbox because the storage memory cap was exceeded before all of the mailboxes or messages could be copied into the portable electronic mail account. On the other hand, where users are charged a premium for additional memory storage, the users are typically unwilling to pay an amount sufficient to cover the direct costs of such storage and/or provide a profit margin for the service provider. Accordingly, neither of these approaches is satisfactory for maintaining a portable electronic mail account.